Lessons learned with vibration monitoring systems in German nuclear power plants

Pergamon

www.elsevier.com/locate/pnucene

Progress in Nuclear Energy, %Iol. 43, No. 1-4, pp. 159-165, 2003

Available on l ine at www.sciencedirect .corn © 2003 Elsevier Science Ltd. All rights reserved _~ Printed in Great Britain

I I C l I N C l l D l l q lC : ' I r , 0149-197(I/03/5 - see front mauer

doh 10.1016lS0149-1970(03)00022-2

L E S S O N S L E A R N E D WITH VIBRATION MONITORING

S Y S T E M S I N G E R M A N N U C L E A R P O W E R P L A N T S

A. KOLBASSEFF, R. SUNDER

Institut fur Sicherheitstechnologie (ISTec) GmbH Forschungsgelande, 85748 Garching, Germany

ABSTRACT Compared to international standards, vibration diagnosis of German pressurised water reactors ranks high (Bastl et al, 1980). Based on the Condition Monitoring System COMOS a lot of operational experiences could be gathered meanwhile and led to founded knowledge of long-term experiences. Meanwhile a successor system called "COMOSnt" is developed and installed in several plants. After a brief description of COMOS system features, a case study based on PWR internal vibrations is presented. In German BWRs, until now no multi-sensorial vibration monitoring comparable to PWR standards is established. Reasons for this can be related to constructional characteristics, differences in neutron-flux instrumentation and variable speed-driven operating mode of reactor recirculation pumps. However, current issues with regard to core internals were reasons to investigate BWR vibration monitoring principles, taking into account e.g. vibration sensors at reactor recirculation pumps, self-powered neutron detectors and accelerometers from loose parts monitoring. Noise analyses of incore-neutron flux signals showing specific modes of fuel assembly vibrations are presented in detail. The correlation analyses could be verified by means of accompanying structure model calculations. At present, specific systems for recirculation pump monitoring are running in three BWRs, an overall vibration monitoring concept including core internals is tested in a reference plant. © 2003 Elsevier Science Ltd. All rights reserved.

KEYWORDS pressurized water reactor (PWR), internal vibrations, vibration monitoring, condition monitoring system, boiling water reactor (BWR), fuel assembly vibration, neutron noise investigation, correlation analyses, automated monitoring system, model calculations

159

160 A. Kolbasseff and R. Sunder

1. PWR VIBRATION MONITORING

As presented during the 1987 SMORN meeting in Munich - COMOS has a modular structure with regard to functionally separated surveillance modes (Van Niekerk F. and Sunder R., 1987). The "Primary System Diagnosis" is based on double-traced amplitude spectra and derived coherence and phase functions. What is monitored are resonances of structures of so-called passive components, like pressure vessel, core barrel, upper core support and piping systems. The "Pump Diagnosis" is also derived from double-traced spectra. Monitored are rotational-frequency and higher-harmonics as well as resonances of pump structures. It is known fact that vibration monitoring provides reliable information for the diagnosis of deficiencies on suspensions of components, wear processes on structural parts and thermal/mechanical fatigue processes or high cycle fatigue. Therefore, the cumulative data of more than 70 operating years of running COMOS systems would also be kept in condensed form in the new COMOSnt systems so that trend representations over desired recording periods can be retrieved. COMOSnt uses system developments that have already been carried out jointly with Schenck Vibro Company in fields of turbo set and recirculation pump vibration diagnosis. COMOSnt modular structure is presented in Fig. la. Monitoring functions are concentrated to reactor coolant pumps, feed water pumps but also to the primary circuit components and the reactor core. Actual vibration behaviour can be compared with reference conditions from many years ago. Based on these considerations, the first case study is concentrated to vibrational characteristics of the so-called secondary core support (SCS) structure (Fig. 2a).

Automated vibration trending for frequency bands of high significance is an important feature of mode 2 analysis with COMOSnt. Derived from a baseline-analysis, relevant resonance peaks in all spectral signatures of each monitored PWR are well known (Fig. 2b). For each signature, up to 8 frequency bands of interest may be specified. Fig. 2c represents spectra cascades of absolute displacement sensor A1 for KKG-PWR over a time span of ten fuel cycles. In each signature, four resonances are included in automated feature trending: core barrel beam mode vibration, RPV pendular vibration, RPV vertical vibration and SCS beam mode vibration. These measurements were done once per week. The corresponding frequency trend curves for fuel cycles 5-14 are shown in the lower part of Fig. 2d. Related to a reference signature taken some weeks after plant start up, the frequency trend of the selected SCS resonance is plotted versus time.

A comparing analysis of the peak structure between 6 th and 14 th fuel cycle is documented in Fig. 2d. Starting with fuel cycle number 13, slight decrease of SCS resonance frequency can be detected. The alert level threshold was set to 29.1 Hz. From this point of view, actions should be aspected during the next two fuel cycles.

Meanwhile we selected further information during additional fuel cycles. Fig. 2f represents the current situation for both vibrational directions of SCS component, whereas the predominant SCS vibrations at 29.3 Hz show a rather stable situation of the observed natural frequency, the above mentioned "stiff" vibration direction shows a distinct and later on smooth deviation from 29.75 up to 29.20 Hz. Since fuel cycle 18 th, stable behaviour can be stated. This behaviour is a well known long term characteristic due to certain consolidation settlement in the course of screw thread.

Based on these interpretations of the observed trend behaviour in KKG-PWR, planned inspection (Fig. 5e) of secondary core support structure could be cancelled up till now, saving cost, time and radiation exposure for the utility people. This example proves the advantage of systematic data storage of long time spans to get statistical founded background information (Sunder R. et al, 1991).

Vibration monitoring systems 161

PWR primary circuit Reactor coolant pumps Further pumps (e.g. feedwater pumps)

I//Z TT Control room

t

BWR rtlclrculaliton

t ~ ~ p u m p casing

BWR recirculation pump shaft

i

Z

TT I ' Control room

I

t

BWR preszure

TT upgraded system

FIGURE 1" Vibration monitoring systems COMOSnt / VIBROCAM 500 ZUP for advanced condition monitoring of PWR / BWR

1 6 2 A , Kolbasseff and R. Sunder

.......... ~ , ~ ......................... ~ ....................................................................................... ~ i i l ̧

i '~ 10 .2

o ~. 10-4

] - ~

10 -6 I

CB beam mode v i b r a t i o n

~L RPV pendul~ v i b r a t i o n

iiii ,~ . , o ~0 ~o ~o 4o \ U J ~ . f r e q u e n c y [Hz] ,

A~

I ~ t - - - " I ! I t

~ 1 I I i i I i I i ~ I I i I i I I

2 8 2 9 3 0 3 1 3 2

f r equency [Hz]

2 9 7 5 - ' ' " 1 9 8 7 t 9 8 8 1 9 8 9 t g 9 0 1991 1 9 9 2 1 9 9 3 1 9 9 4 1 9 9 5 1 9 ~ i

core . . . . . • i . ! i . : ! i • .

i l !~ ~ i i~!!~!~!!~!~i i~i~;~i~i~/~o~®~ii~i i i~i~ ~ ~!ii~i i ii~i;~ i i : i~ i~ i~y~i~ "~!~*~:~i~!~i!i~i~;i ~̧

, ............... tlt.I " ' ! 1 1 . . . . . . . . . . . . . . l I m l H I l l l l H l l l H I l

. . . . . . . i i ~ 2 3 4 s i 7 s i 1o 11 lZ ~3 ~4 t l ~6 ~7 s

i f f ~ core ¢yck) number

F I G U R E 2: Vibrational behaviour of P W R internals -

Case study of Secondary Core Support Structure

Vibration monitoring systems 1,63

2. BWR VIBRATION MONITORING

In German BWRs, there is no multi-sensorial vibration monitoring installed that is adequate to PWR standards, proven from a safety-related point of view. Reasons for this can be found in the constructional characteristics (more flexible plate structures, complex mounting and bearing conditions of the core internals), differences in the neutron-flux instrumentation (no external neutron chambers available, dynamic part of the power distribution signals affected by boiling process) and a large spectrum of vibratory excitations (variable speed-driven operating mode of the reactor recirculation pumps). However, current issues with regard to core internals (e.g. core shroud integrity) and fuel assemblies (increased coolant flow rate due to new fuel element designs) suggest that the possibilities of a BWR vibration monitoring should be investigated once again, also taking into account several vibration sensors meanwhile installed at the reactor recirculation pumps, which should be well suited for this task. Based on similar functional issues to COMOS system, a recirculation pump monitoring system was designed, specified and meanwhile installed in three BWRs of so-called series '69. A block schematic of this system is documented in Fig. lb (Stabel N. et al, 1999). Moreover, neutron flux power distribution detectors are available and should be involved in these monitoring tasks (Fig. 3a). Starting these investigations, signals of 36 power distribution detectors were recorded each at two 900 MW BWRs during different power levels. First correlation analyses of in-core neutron flux signals showed, that during reduced load operation good conditions are given for the detection of fuel assembly vibrations, in-core tube vibrations, and for the determination of the coolant velocity distribution in the core. During minimum load operation, however, the neutron flux signal is smaller than the noise background. These correlation analyses by the neutron flux signals could be verified by means of accompanying structure model calculations. This is shown by the highly agreement between the calculation and the measuring results. By this, the interpretations of discrete frequency peaks required for the monitoring tasks were obtained. As included in Fig. lb, the calculated natural frequencies of fuel assemblies from type SVEA96 match quite well with the yellow marked frequency bands in in-core neutron noise spectra. The model calculations were done in a very detailed manner (Altstadt E. and M6ssner Th. 1999).

The results were tested during start-up measurements in KKB-nuclear power plant during 14 th and 15 th fuel cycle (Figs 3e/3f). The corresponding spectra cascades over several days period starting from zero power up to nominal power operation of the plant is documented for one SPN detector in Fig. 3d.

Due to a systematic change in core design, switching from one fuel assembly type (SVEA68) to another one (SVEA96), the distribution of fuel assembly types varies from one core to the other. Therefore the mean value of the fundamental fuel assembly natural frequency shifts from 3.5 Hz to 3.3 Hz - dependent on the changed stiffness of different fuel assembly types.

This situation is highlighted in Figs 3g/3h comparing the BOC14 / BOC15 measurements. The fuel assembly distribution in the surrounding of the selected sensor is documented in the inserted scheme. These findings could be proved all over the core, taking into account all relevant sensor combinations. In Fig. 3i, SPN detectors of two selected strings are compared, underlining these findings. As a result of the encouraging results, we installed an upgraded monitoring system for BWRs just some weeks ago including various types of sensors. Acoustic signals, self-powered neutron detectors from the core area, shaft vibra- tion sensors from reactor recirculation pumps and corresponding casing vibration sensors at the motor support of these pumps. Due to this instrumentation, a total of up to 150 measuring values are available (120 SPNDs, 8 acoustic signals, 16 shaft vibration signals and 8 casing vibration signals). In Fig. lb these additional possibilities are included. The advantage of this solution is that all sensor types are still available on-site. There is no need for further investments.

1 6 4 A. Kolbasseff a~M R. Sunder

. . . . . . . . k . . . . . .

embly

I! a ,, SvEA-~

i

, k b j ~ ' ° ; . o L y , . ; I . . . . . . . . .

measurement during power Increase

<

~eJ , ~ z - , ~ . . . . ~.1 I

~ 1 ~ t - ± . . . . . . . . . i " _ _ , _ _ , , ' _ _ _ , _ _ _ _ _ ' _ _ _ . . . . . . . _ , ~

..................... ~:~ .~."~"" .......... ~ ............. . ~ ................. ~ ' ~

"~g~)~ " ~ . . . . . . ~ i'i i~

~ ~ ; k ̧ ~ ;u t t : 4 : ; ~ i i ~ t t ~ u d,~;~ ~ht~ m ~

14, cote cycle

15. core cycle

. . . . u , . ! i i

' i . . . . . . . . , o ; ~ , . . . . .

FIGURE 3: Vibrational behaviour of BWR internals - Case study of fuel assembly vibrations (type SVEA 96)

Vibration monitoring systems 165

3. CONCLUSION

More detailed signal analysis and continuous condition monitoring techniques originally developed for and applied to key components of NPPs like reactor internals and primary circuit components have been successfully developed for improved condition monitoring of rotating machinery. Up-to-date information processing technology and computer systems allow the realisation of very powerful continuous monitoring systems also for complex mechanical components under economically acceptable conditions. By combining well-experienced methods with frequency signal analysis by means of online systems and data link to a diagnosis center where short-term support can be given, a new quality of condition monitoring and failure diagnosis for nuclear power stations is available. When such methods are systematically applied in the plants, a big step could be done towards improvement of maintenance strategy.

REFERENCES

Altstadt E. and MOssner Th. (1999), Finite-Elemente-Berechnungen ftir ein Brennelement des Typs SVEA96, Ergebnisbericht des Instituts fiir Sicherheitsforschung, FZR, Rossendorf

Bastl W., Sunder R. and Wach D. (1980), On-line vibration monitoring of PWR internals, ANS/ENS Topical Meeting on Thermal Reactors Safety, Knoxville

Stabel N., Kolbasseff A. and UBkilat B. (1999), Diagnostische 13berwachung yon Zwangsumw~ilzpumpen an Siedewasserreaktoren deutscher Kernkraftwerke, VDI-Schwingungstagung, Mannheim

Sunder R., Baleanu M., Kieninger K., Kolbasseff A., Kuhn W. and ROsier H. (1991), Experiences and results with COMOS - an on-line vibration analysis and monitoring system, Proceedings of SMORN VI, Vol. 1, pp. 5.01-5.12

Van Niekerk F. and Sunder R. (1987), COMOS - an on-line system for problem orientated vibration monitoring, Proceedings of SMORN V, Progress in Nuclear Energy 21, pp. 155-171